1. Field of the Invention
The present invention relates to a video information transmission system, and an apparatus and a program used for the video information transmission system. More specifically, the present invention relates to a video information transmission system, and an apparatus and a program used for the video information transmission system for efficiently transmitting video information such as a live video at real time while suppressing the degradation of video quality by controlling congestion on a network without QoS (Quality of Service) guarantee.
2. Description of the Related Art
On a QoS non-guaranteed IP network such as the Internet, a transmission bandwidth, a transmission error rate, a jitter and the like change with the passage of time. Due to this, if continuous media such as video or voice information are compression-transmitted and continuously reproduced, the degradation of quality occurs, making it often difficult to smoothly reproduce the continuous media.
FIG. 14 is a functional block diagram showing a conventional video information transmission apparatus. In FIG. 14, on a sender side, video information input from a camera 141 is subjected to a sequential compression processing by a real-time encoder 142 at a fixed transmission bit rate, packetized by a transmission control section 143, and output to an IP network 140.
On the other hand, on a receiver side, packets from the network 140 are received by a reception control section 144 at real time, reassembled, and decoded by a real-time decoder 145, and reproduced and displayed by a reproducing and display section 146.
In this case, even if the state of the network 140 changes and the bandwidth of the network 140 narrows, the sender side continues to transmit data at the fixed transmission bit rate equal to or larger than the bandwidth. As a result, network congestion occurs, packet propagation delay grows and packet loss is induced, so that the degradation of reproduction quality eminently appears on the receiver side.
There have been proposed measures to deal with these disadvantages by decreasing a transmission bit rate in accordance with an excess rate if a packet loss rate on a receiver side exceeds a certain threshold as described in “J. C. Bolt and T. Turletti, “Scalable feedback control for multicast video distribution in the Internet”, ACM Proc. Sigcomm, pp. 58-67, London, UK, September 1994”, and “I. Busse, B. Deffner, H. Schulzrinne, “Dynamic QoS Control of Multimedia Application based on RTP”, ACM Computer Communications, June 1996”. However, these measures have a disadvantage in that the transmission bit rate is not controlled before packet loss occurs and reproduction quality degrades. Further, if the packet loss is attributed to buffer overflow at a gateway on the network, the transmission bit rate is increased until the buffer on the network becomes full. Due to this, these measures have a disadvantage in that delay by as much as the size of the network buffer always occurs.
Moreover, there have been proposed measures by feeding back the queue length of a network switch, predicting the increase/decrease of transmission delay and controlling congestion as described in “H. Kanakia, P. Mishra, and A. Reibman, “An adaptive congestion control scheme for real-time packet video transport”, ACM Proc. Sigcomm, pp. 20-31, September 1993”. However, the measures require a mechanism in which each network switch feeds back queue length information, disadvantageously complicating packaging. Besides, the measures have a disadvantage in that data transmission cannot be realized using a standard protocol such as RTP (Real-time transport protocol)/RTCP (RTP control protocol) standardized under IETF RFC1889.
Furthermore, there have been proposed measures by setting a network buffer value to be close to a target value and thereby controlling congestion and fixing transmission delay as described in “Yano, et al., “Rate control method based on roundtrip time suited for real-time transfer of continuous media”, IN98-23, CS98-23, MVE98-23, pp. 85-90, April 1998”. However, the measures have disadvantages in that not only the optimum setting of a target buffer value is difficult but also the re-setting of the target value is necessary whenever the bandwidth of the network changes. Further, the measures have a disadvantage in that if the target buffer value is controlled to be fixed and the network bandwidth narrows by half, for example, delay doubles.